1. Field of the Invention
The present invention relates to method of controlling a machining center having a large number of tools of plural sorts using a computerized numerical controller (CNC), and more particularly to method of detecting life of every tool in the control system.
2. Description of the Prior Art
Cutting edge of a tool used in a machine tool is abraded as machining is performed for a long time, thereby the cutting quality is deteriorated. Abrasion of the cutting edge decreases the machining efficiency and affects adversely to the machining accuracy of surface of a work. In order to eliminate such disadvantages life of each tool must be supervised separately. In a method known as a conventional control method for tool life, time relating to cutting by a tool is integrated, and when the integrated value becomes a limit use life time which is empirically set from tool use conditions, information is performed regarding that the tool is at the limit life state. This system may be used for detecting the tool life in an exclusive machine tool in which specified tools are used exclusively and therefore sorts of the tools are limited. In a numerically controlled machining center with automatic tool change device, however, many sorts of tools are frequently changed and these are used uncontinuously. If above-mentioned method of integrating the use time by timers is applied to such machining center, the timers in number corresponding to sorts of the tools will be required. Moreover, one timer corresponding to a tool relating to machining must be selected among a plurality of timers, and the selected timer must be operated simultaneously at the start of machining and stopped simultaneously at the end thereof. Accordingly, control circuit for changing the operation becomes quite complicated and enlargement of the control apparatus results in high cost, thus the method has many problems for practicable use.
In order to eliminate these disadvantages, method of discriminating the life end of each tool is proposed. The method comprises steps of reading the tool number to be used in the machining, relative speed of machining and feed amount from the control code on numerical control program (hereinafter referred to as "NC program"), calculating time for the tool to perform the machining in a machining routine, and accumulating the machining time of every tool and comparing the cumulative value with the life time given previously to each tool. This method can be realized by software stored in a computerized numerical controller (CNC).
Any of above-mentioned control methods decides the tool life based on the cumulative machining time. However, degree of abrasion of a tool varies depending on that the cutting rate is high or low even if the same work is machined by the same tool in the same use time. That is, degree of abrasion increases as the cutting rate becomes high. Consequently, if the tool life is determined on the basis of only the tool use time, a tool in use at relatively slow cutting speed may be discriminated to come to the limit of use life although not coming thereto. On the contrary, a tool in use at high cutting speed may be used over the limit life, which causes deterioration in the cutting efficiency and the precision of cutting surface.